I've used dried lime powder (a common ingredient in Persian cooking) as a substitute before. If you have access to a Middle Eastern grocery store, it's usually available there. It's sometimes also labeled as dried lemon powder or limoo amani. It's got the the same sour quality as amchoor and won't add extra liquid to the recipe.

Aamchur is used for its tartness in indian cuisine.ts basically a powder of sun dried raw mangoes, it tends to have an earthy component in addition to the sour notes. the closest in texture would be dried pomegranate powder, (which is not as commonly available as aamchur), but as Kitchen butterfly said, lime/lemon or especially tamarind juice is a great substitute.

Amchoor powder is ground green dried mango powder, often used in Indian cuisine. It is has a sour tangy fruity flavor. Some suggest substituting lime or lemon juice or tamarind juice or pulp